The Right CMOS Camera for the Job
Manufacturers must consider the trade-offs in the different levels of sensor integration
to offer the right CMOS camera for the application.
While the CCD offers
the best sensor performance for digital imaging, the CMOS image sensor is superior
in terms of integrating functionality on-chip. Even the much-touted camera-on-a-chip
is not beyond the capabilities of CMOS technology, promising sensors that incorporate
drivers, power supplies, timing generators and even correlated double-sampling units,
analog-to-digital converters and digital signal processors on-chip. But just because
certain circuitry can be integrated onto a sensor chip, it does not follow that
is always advantageous to do so. Therefore, the optimal degree of integration for
today’s — as well as tomorrow’s — applications will continue
to differ according to the desired application.
CMOS camera manufacturers must consider the target application when
deciding how much integration is necessary. Although additional circuitry can be
integrated onto the sensor, it is not necessarily advantageous to do so. Photos
courtesy of Eastman Kodak Co.
The so-called single-chip camera offers the physically
smallest digital camera design and the lowest possible cost and parts count, but
there are limitations to its performance. Moreover, its economic and real estate
advantages are not as great as they at first may appear.
For both analog-to-digital conversion
and digital signal processing, the expertise of the companies that specialize in
these areas typically exceeds that of sensor manufacturers, and the discrete integrated
circuits typically have tighter linewidth geometries (0.13 to 0.18 μm) than
the sensor makers’ integrated imagers (0.35 to 0.5 μm). It is possible,
of course, to shrink the geometries of image sensors, but this tends to diminish
their performance and image quality, particularly with respect to dark current (higher
noise) and charge capacity.
Quality consumer cameras
will employ a two-chip design, combining an integrated CMOS sensor and a discrete
digital signal processor.
An integrated analog-to-digital converter
or digital signal processor always requires less space than a discrete one but is
not necessarily less expensive. The dense integration of discretes will keep the
die size small on the silicon wafer, yielding major cost savings as well as minimizing
the required space for the packaged component. On the other hand, adding functions
to sensor chips increases their die size and compromises their real estate and cost
For applications that demand the highest
performance and image quality, it is advantageous to implement certain functions
with discrete, state-of-the-art integrated circuits. The best design for quality
consumer cameras, for example, will be a two-chip device combining an integrated
CMOS sensor that contains all the functionality on-chip for acquiring an image and
a discrete digital signal processor that offers color correction, white balancing
and such signal-processing functions as noise filtering and, perhaps, defect correction.
“Prosumer” and professional applications will require a three-chip design,
comprising an integrated CMOS sensor, a discrete digital signal processor and, to
attain the best noise performance, a discrete analog-to-digital converter.
Single-chip CMOS cameras will find
applications in cellular phones and in a variety of other handheld consumer and
communications electronic devices into which digital imaging will be integrated,
serving where size, and not quality, is the primary concern. For applications where
image quality is the thing, the future belongs to multichip CMOS designs and —
at the highest end of the market — multichip designs based on CCDs.
Meet the author
Helen Titus is marketing manager at Eastman Kodak
Co., Image Sensor Solutions, in Rochester, N.Y. She holds a master’s and a
bachelor’s degree in electrical engineering from?Rochester Institute
of Technology and from Gannon University in Erie, Pa., respectively
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